Implantable medical devices (IMD) that can alert patients or third parties to the detection of ischemic events including heart attacks (Myocardial Infarction) can save lives and reduce damage to a patient's heart tissue, improving the post-myocardial-infarction quality of life. Myocardial infarction (MI) occurs when a blood clot blocks the blood supply to a portion of the heart causing the heart tissue to become hypoxic (ischemic) and also experience decreased metabolite removal. Ischemia detection may occur by analysis of the patient's cardiac activity, especially via electrical waveforms sensed by electrodes.
The sino-atrial (SA) node normally serves as the pacemaker for the healthy human heart. Electrical pacing pulses travel through conduction pathways and cause electrical/chemical changes in adjacent myocardium, producing the coordinated activity of various regions which are associated with the beating of the heart and normal blood-flow. When the electrical conduction network of the heart becomes diminished, when there are factors which cause tissue damage, or when there is insufficient blood-flow supplying the heart itself, the timing of the propagating electrical signals will cause the heart to beat in a less efficient manner, to beat abnormally, or to not beat at all. The heart has back-up systems for beating when the SA node experiences failure, such as the atrioventricular node (AV node), which has a natural pacing rate that is slower than the SA node. Although the heart can somewhat compensate for abnormal pacing, providing artificial pacing in order to assist a sick heart deters the risk of further complications which may lead a patient to more quickly experience worsening levels of heart failure.
Most pacemakers are “demand pacemakers” which utilize sensing in order to only deliver pacing when the heartbeat is too slow. This feature allows pacing to not occur continuously and there may be both paced and non-paced heartbeats throughout the day. Pacemakers can also be “rate-responsive” and contain means for determining what the heart rate should be at different moments in time. Not only do they provide pacing when the heart rate drops below a selected minimum, but these can also set the rate of pacing to create a well adjusted heart rate. Rate-responsive pacemakers may rely upon multiple technologies and sensors to determine the appropriate heart rate. An accelerometer may serve as an activity sensor which detects the patient's level and direction of movement so that increased pacing rates may be provided when the patient is active. Additionally, various sensors may serve to measure breathing characteristics, such as rate, so that faster breathing will provide rate-responsive pacing more akin to that seen endogenously in patients with normal cardiac electrical systems. Additional sensors that measure indices such as carbon-dioxide or cardiovascular sounds can also be used to adjust pacing rate.
Pacemakers can have two or more stimulation leads which not only keep the heart rate from dropping too low, but also to maintain improved coordination between different chambers of the heart. For example, the atria and the ventricles can be made to cooperate better by pacing the structures sequentially and at a well chosen latency for a particular heart rate range. In a multiple lead pacemaker, the processor analyzes sensed cardiac data, and derives information that enables the pacemaker to determine if when, and where to provide pacing to improve synchronization of the different chambers.
One type of multiple lead pacemaker is a “biventricular pacemaker”, also known as CRT (cardiac resynchronization therapy) device. CRT devices can pace both the septal and lateral walls of the left ventricle in order to “resynchronize” a heart. A CRT device may utilize a first lead located in the right ventricle to provide septal stimulation while the second is routed through the coronary sinus and anchored to pace the lateral wall of the left ventricle. A third lead, positioned in the right atrium, may also be used to improve ventricular synchrony with the atrial contraction. The timing between the atrial and ventricular contractions, as well as between the septal and lateral walls of the left ventricle can be programmably adjusted to improve cardiac function in individual patients. CRT technology can be implemented within an implantable cardioverter-defibrillator (ICD). Medically dangerous rhythms such as certain arrhythmias can be halted by giving the heart an electric shock delivered using the ICD (e.g., within the heart itself or by stimulation sites in the chest wall) or external devices. Modern programmable pacemakers allow the medical personnel to select the optimum pacing modes for individual patients. The term “pacemaker” shall henceforth mean a system of one or more devices that provides one or more of pacemaker, CRT, cardioversion, and defibrillator capacity.
When therapies, such as electrical pacing of the patient's heart, occur concurrently with ischemia monitoring of the patient, the effects of these therapies both on the heart and on the sensed cardiac data must be managed by monitoring methods so that ischemia can be accurately measured from the sensed cardiac data.
The combination of a pacemaker or ICD with an ischemia detector is described by Fischell et al in U.S. Pat. Nos. 6,112,116, 6,272,379 and 6,609,023. Fischell describes an IMD which can detect a change in the electrical signal from the patient's heart (cardiac electrical signal) that is indicative of a cardiac event, such as an acute ischemia, and then provide a notification of such an event. The IMD can also be a medical device which senses and/or stimulates cardiac, neural, vagal-nerve, or other anatomical target in order to control cardiac activity. Fischell also describes an external alarm system that can provide additional visual, sonic and vibratory alerting signals and may also provide voice/data communication between the IMD and a remote medical monitoring station.